Dielectric ceramic, method for producing the same, laminated ceramic electronic element, and method for producing the same

ABSTRACT

A dielectric ceramic which exhibits small variation in dielectric constant, allows use of a base metal, can be fired in a reducing atmosphere and which is suitable for constituting a dielectric ceramic layer for, e.g., a laminated ceramic capacitor is obtained by firing barium titanate powder in which the c-axis/a-axis ratio in the perovskite structure is about 1.000 or more and less than about 1.003 and the amount of OH groups in the crystal lattice is about 2.0 wt. % or less. The barium titanate powder starting material preferably has a maximum particle size of about 0.3 μm or less and an average particle size of about 0.05-0.15 μm. Each particle of the barium titanate powder preferably comprises a low-crystallinity portion and a high-crystallinity portion, the diameter of the low-crystallinity portion being about 0.5 times or more the particle size of the powder. When sintered, the powder satisfies the ratio of (average grain size of the fired dielectric ceramic)/(average particle size of barium titanate powder starting material), R, of about 0.90-1.2, to thereby suppress considerable grain growth. A laminated ceramic electronic element including a laminate of a plurality of layers of the above-mentioned dielectric ceramic, as well as a method for producing the same, is described.

[0001] This is a divisional of U.S. patent application Ser. No.09/234,356, filed Jan. 20, 1999 in the name of Nobuyuki WADA, TakashiHIRAMATSU, Jun IKEDA and Yukio HAMAJI.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a dielectric ceramic which isadvantageously used in a laminated ceramic electronic element such as alaminated ceramic capacitor having an internal conductor formed of abase metal such as nickel or nickel alloy, and to a method for producingthe dielectric ceramic. The present invention also relates to alaminated ceramic electronic element which is formed of the dielectricceramic and to a method for producing the same.

[0004] 2. Description of the Related Art

[0005] Miniaturization and cost reduction of laminated ceramicelectronic elements is in progress. For example, the ceramic layer hasbeen thinned and a base metal has been employed as an internal conductorin such a ceramic electronic element. In the case of a laminated ceramiccapacitor, which is one type of laminated ceramic electronic element,the dielectric ceramic layer has been formed as thin as about 3 μm and abase metal such as Cu or Ni has been employed as a material forproducing an internal conductor, i.e., an internal electrode.

[0006] However, when the ceramic layer becomes thin, the strength of anelectric field applied to the layer increases and causes a problem inthe ceramic layer dielectric exhibits a great change in dielectricconstant induced by the electric field. Decrease of the size of ceramicgrains in the thickness direction of the ceramic layer also causes aproblem in reliability.

[0007] In order to cope with such situations, Japanese PatentApplication Laid-Open (kokai) Nos. 9-241074, 9-241075, etc. haveproposed ceramic materials which enable enhanced reliability byincreasing the size of ceramic grains in the thickness direction of thedielectric ceramic layer. Thus, controlling the grain size of ceramicgrains allows a reduction in change of dielectric constant induced by anelectric field or temperature.

[0008] However, when a barium titanate material exhibiting a strongdielectric property is used as a material for a dielectric ceramic layerhaving a thickness of about 1 μm or less in the above-describedconventional art, the effect of electric field intensity on thedielectric ceramic layer increases to thereby lower the dielectricconstant considerably. When a laminated ceramic electronic element isconstructed thereof, the rated voltage must be lowered. Therefore,realization of a thin layer having a thickness as thin as 1 μm or lessis difficult or impossible so long as the above-described conventionalart is employed to solve the problem.

SUMMARY OF THE INVENTION

[0009] In view of the foregoing, the present invention is directed to adielectric ceramic which is advantageously used in a laminated ceramicelectronic element including a thin ceramic layer having a thickness asthin as about 1 μm or less and to a method for producing the dielectricceramic. The present invention is also directed to a laminated ceramicelectronic element which is formed of the dielectric ceramic and to amethod for producing the same.

[0010] In one aspect of the present invention, there is provided adielectric ceramic which is obtained by firing a barium titanate powderhaving a perovskite structure in which the c-axis/a-axis ratio in theperovskite structure is about 1.000 or more and less than about 1.003and the amount of OH groups in the crystal lattice is about 2.0 wt. % orless.

[0011] In another aspect of the present invention, there is provided amethod for producing the dielectric ceramic, which method comprises thesteps of providing a barium titanate powder in which the c-axis/a-axisratio in the perovskite structure is about 1.000 or more and less thanabout 1.003 and the amount of OH groups in the crystal lattice is about2.0 wt. % or less; and firing the barium titanate powder.

[0012] The amount of OH groups is determined based on the loss at 150°C. or more as measured during thermogravimetric analysis of specimens.

[0013] The barium titanate powder preferably has a maximum particle sizeof about 0.3 μm or less and an average particle size of about 0.05-0.15μm.

[0014] Also, each particle of the above-described barium titanate powderpreferably comprises a low-crystallinity portion and ahigh-crystallinity portion, and the diameter of the low-crystallinityportion is preferably about 0.5 times or more the particle size of thepowder. As shown in FIG. 1, which is a transmission electron microscopicphotograph of barium titanate powder, and FIG. 2, which is anexplanatory sketch therefor, the term “low-crystallinity portion” 21used herein refers to a domain containing a number of lattice defectssuch as a void 22, whereas the term “high-crystallinity portion” 23 usedherein refers to a domain containing no such lattice defects.

[0015] Also, when the ratio (average grain size of fired dielectricceramic)/(average particle size of provided barium titanate powder) isrepresented by R, R preferably falls within the range of about 0.90-1.2.

[0016] Grains that constitute the dielectric ceramic of the presentinvention may have a core-shell structure in which the composition andcrystal system differ between the core and the shell or a homogeneousstructure having a uniform composition and crystal system.

[0017] The term “crystal system” used herein refers to a crystal systemof perovskite crystals, i.e., to a cubic system having a c-axis/a-axisratio in the perovskite structure of about 1 or to a tetragonal systemhaving a c-axis/a-axis ratio in the perovskite structure of about 1 ormore.

[0018] In yet another aspect of the present invention, there is provideda laminated ceramic electronic element including a laminate formed of aplurality of ceramic layers and an internal conductor formed along aspecific interface between adjacent dielectric ceramic layers.

[0019] Specifically, in the present invention, the dielectric ceramiclayer included in the laminated ceramic electronic element isconstituted by a dielectric ceramic obtained by firing a barium titanatepowder having a perovskite structure in which the c-axis/a-axis ratio inthe perovskite structure is about 1.000 or more and less than about1.003 and the amount of OH groups in the crystal lattice is about 2.0wt. % or less.

[0020] In the above-described laminated ceramic electronic element, theinternal conductors preferably contain a base metal such as nickel ornickel alloy.

[0021] The laminated ceramic electronic element may further include aplurality of external electrodes at different positions on a side faceor faces. In this case, the internal conductors are formed such that oneend of each of the internal conductors is exposed to the side face so asto be electrically connected to one of the external electrodes. Such astructure is typically applied to laminated ceramic capacitors.

[0022] In a still further aspect of the present invention, there isprovided a method for producing a laminated ceramic electronic element,which method comprises the steps of providing a barium titanate powderin which the c-axis/a-axis ratio in the perovskite structure is about1.000 or more and less than about 1.003 and the amount of OH groups inthe crystal lattice is about 2.0 wt. % or less; fabricating a laminatein which a plurality of ceramic green sheets containing the bariumtitanate powder and internal electrodes are laminated so that theinternal electrodes are present along specific interfaces of the ceramicgreen sheets; and firing the barium titanate powder to thereby provide adielectric ceramic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Various other objects, features, and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood with reference to the following detaileddescription of the preferred embodiments when considered in connectionwith the accompanying drawings, in which:

[0024]FIG. 1 is a photograph of barium titanate powder provided forproducing a dielectric ceramic according to the present inventionobtained by transmission electron microscopy;

[0025]FIG. 2 is an explanatory sketch of the electron microscopicphotograph shown in FIG. 2; and

[0026]FIG. 3 is a cross-sectional view showing a laminated ceramiccapacitor 1 according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The barium titanate powder used in the present invention has acomposition represented by formula:(Ba_(1-x)X_(x))_(m)(Ti_(1-y)Y_(y))O₃. The composition is not furtherlimited specifically. X may comprise Ca, single species of rare earthelements, and a combination of two or more thereof. Y may comprisesingle species such as Zr or Mn or a combination of two or more speciesthereof. In general, m is preferably about 1.000-1.035, depending on thecomposition of the barium titanate powder, in order to obtain anon-reducing dielectric ceramic.

[0028] The barium titanate powder which is advantageously used has aperovskite structure in which the c-axis/a-axis ratio is about 1.000 ormore and less than about 1.003. Moreover, the amount of OH groups in thecrystal lattice is about 2.0 wt. % or less; the maximum particle size isabout 0.3 μm or less; and the average particle size is about 0.05-0.15μm. Such a barium titanate powder can be obtained by thermally treatingbarium titanate powder which is produced through a wet synthesis methodsuch as a hydrothermal synthesis method, a hydrolysis method, or asol-gel method. There may also be employed a solid phase synthesismethod in which a carbonate, an oxide, etc. of elements constituting thebarium titanate powder are mixed and thermally treated.

[0029] In the above-described thermal treatment, the conditions formoderate grain growth are selected so as to realize a c-axis/a-axisratio of about 1.000 or more and less than about 1.003. For example, thetreatment is performed in air or performed in a nitrogen stream or H₂Ostream by controlling the temperature and time of the treatment. When asolid-phase method is used, because the c-axis/a-axis ratio mightdecrease depending on conditions for disintegration of the synthesizedpowder, the disintegration conditions must be controlled.

[0030] The diameter ratio of the low-crystallinity portion 21, i.e., theratio of the diameter of the low-crystallinity portion 21 to theparticle size of the powder, after the above-described thermaltreatment, is predetermined to be about 0.5 or more in each particle ofthe barium titanate powder shown in FIG. 1 and FIG. 2. Such a diameterratio may be obtained through thermal treatment at a temperatureelevation rate of 5° C./min or more.

[0031] The relationship between the average particle size of thethus-provided barium titanate powder and the average grain size of thefired dielectric ceramic, i.e., the ratio of (average grain size offired ceramic)/(average particle size of provided barium titanatepowder), which is represented by R, is preferably about 0.90-1.2.Briefly, considerable grain growth is preferably prevented duringsintering for producing ceramics. For example, for this purpose an Mncomponent and/or an Mg component, an Si-base sintering aid, etc. areadded to barium titanate powder. In general, these additives may beincorporated into barium titanate powder in the form of an oxide powderor carbonate powder. Alternatively, there may be used a method in whichbarium titanate powder is coated with a solution containing theseadditives and then thermally treated.

[0032] Such barium titanate powder is fired to thereby produce adielectric ceramic, which is used in a laminated ceramic electronicelement, e.g., a laminated ceramic capacitor 1 illustrated in FIG. 3.

[0033] As shown in FIG. 3, the laminated ceramic capacitor 1 comprises alaminate 3 containing a plurality of laminated dielectric layers 2 and afirst external electrode 6 and a second external electrode 7 which areprovided on a first side face 4 and a second side face 5 of the laminate3, respectively. The laminated ceramic capacitor 1 in its entiretyconstitutes a rectangular parallelepiped-shaped chip-type electronicelement.

[0034] In the laminate 3, first internal electrodes 8 and secondinternal electrodes 9 are alternately disposed as internal conductors.The first internal electrodes 8 are formed along specific interfacesbetween dielectric ceramic layers 2 such that one end of each of theinternal electrodes 8 is exposed to the first side face 4 of thelaminate 3 so as to be electrically connected to the first externalelectrode 6, while second internal electrodes 9 are formed alongspecific interfaces between dielectric ceramic layers 2 such that oneend of each of the internal electrodes 9 is exposed to the second sideface 5 of the laminate 3 so as to be electrically connected to thesecond external electrode 7.

[0035] In the laminated ceramic capacitor 1, the dielectric ceramiclayers 2 included in the laminate 3 comprise the above-mentioneddielectric ceramic.

[0036] In order to produce the laminated ceramic capacitor 1, there areprovided starting materials comprising a primary component such asbarium titanate and an additive to improve characteristics andsinterability. The materials are weighed in predetermined amounts andwet-mixed to form a mixed powder.

[0037] Then, an organic binder and a solvent are added into the mixedpowder to thereby obtain a slurry, and a ceramic green sheet forming thedielectric ceramic layer 2 is produced by use of the slurry.

[0038] Subsequently, electrically conductive paste films forminginternal electrodes 8 and 9 are formed on the specific ceramic greensheets. The conductive paste film contains a base metal such as nickelor copper, or an alloy thereof, and is formed through a method such asscreen printing, vapor deposition, or plating.

[0039] A plurality of ceramic green sheets, including those on whichconductive paste film has been formed as described above, are laminated,pressed, and cut if necessary. Thus, there is produced a green laminate3 in which ceramic green sheets and the internal electrodes 8 and 9formed along specific interfaces between ceramic green sheets arelaminated, one end of each of the internal electrodes 8 and 9 beingexposed to the side face 4 or 5.

[0040] The laminate 3 is then fired in a reducing atmosphere, to therebytransform barium titanate powder into the dielectric ceramic. In thisstep, the above-described grain size ratio R is controlled so as to fallwithin the range of 0.90≦R≦1.2.

[0041] The first external electrode 6 and the second external electrode7 are formed on the first side face 4 and on the second side face 5 ofthe laminate 3, respectively, so as to contact with the exposed ends ofthe first internal electrodes 8 and second internal electrodes 9 in thefired laminate 3.

[0042] No particular limitation is imposed on the composition of thematerials for producing the external electrode 6 and 7. Specifically,there may be used the same materials as those of the internal electrodes8 and 9. The external electrodes may also be constructed of a sinteredlayer comprising electrically conductive metal powder such as powder ofAg, Pd, Ag—Pd, Cu or a Cu alloy; or a sintered layer comprising theabove conductive metal powder blended with glass frit such asB₂O₃—Li₂O—SiO₂—BaO, B₂O₃—SiO₂—BaO, Li₂O—SiO₂—BaO or B₂O₃—SiO₂—ZnO. Thecomposition of the materials for producing the external electrode 6 and7 is appropriately determined in consideration of factors relating tothe laminated ceramic capacitor 1 such as use or environment of use.

[0043] As described above, the external electrodes 6 and 7 may be formedby applying the metal powder paste forming them on the fired laminate 3and burning. The electrodes may also be formed by applying the paste onthe unfired laminate 3 and burning simultaneous with firing the laminate3.

[0044] The external electrodes 6 and 7 may be coated with plating layers10 and 11 formed of Ni, Cu, an Ni—Cu alloy, etc., respectively, inaccordance with need. The plating layers 10 and 11 may further be coatedwith second plating layers 12 and 13 formed of solder, tin, etc.,respectively.

[0045] The present invention will next be described in detail by way ofexamples, which should not be construed as limiting the invention.

EXAMPLES

[0046] The laminated ceramic capacitor produced in this Example is alaminated ceramic capacitor 1 having a structure shown in FIG. 3.

[0047] Different barium titanate materials having compositions shown inTable 1 were prepared by hydrolysis. Barium titanate materials having acalcium content as high as 10 mol % were prepared by mixing bariumtitanate prepared by hydrolysis and calcium titanate materials preparedby hydrothermal synthesis (see H and I shown in Table 1). The resultantmaterial powders have a particle size of 50 to 70 nm and a cubicstructure containing many OH groups in lattices of a perovskitestructure. Through heat-treatment of these materials under a variety ofconditions in an atmosphere of air, barium titanate powders A to Nhaving different “c/a” values (c-axis/a-axis ratio), average particlesizes, maximum particle sizes, amounts of OH groups and diameter ratioswere prepared. Aggregations produced during heat-treatment weredisintegrated after heat-treatment. TABLE 1 Aver- Maxi- age mum Amountparticle particle of OH Dia- BaTiO₃ (Ba_(1−x)Ca_(x))_(m)TiO₃ size sizegroups meter powder x m c/a (μm) (μm) (%) ratio A* 0.00 1.005 1.001 0.080.20 2.24 0.9 B  0.00 1.010 1.002 0.10 0.20 1.40 0.8 C  0.00 1.010 1.0020.14 0.27 1.05 0.5 D  0.00 1.015 1.002 0.13 0.28 1.23 0.6 E* 0.00 1.0151.002 0.14 0.38 0.82 0.6 F* 0.00 1.015 1.004 0.25 0.35 0.60 0.3 G  0.051.010 1.000 0.07 0.20 1.75 0.9 H  0.10 1.010 1.002 0.14 0.27 0.96 0.5 I*0.10 1.010 1.004 0.25 0.38 0.65 0.3 J* 0.10 1.010 1.002 0.13 0.25 0.920.4

[0048] The “c/a” values shown in Table 1 were determined by X-raydiffraction of barium titanate powders. That is, the results obtainedfrom X-ray diffraction were subjected to X-ray profile fitting usingRietveld analysis to precisely determine lattice constants. The averageparticle size and the maximum particle size were measured by observationof barium titanate powders under a scanning electron microscope. Theamount of OH groups was measured by way of a loss of weight at atemperature of 150° C. or higher as measured by thermogravimetricanalysis of barium titanate powders.

[0049] The diameter ratio shown in Table 1 is a ratio of the diameter ofthe low-crystallinity portion to the particle size of the powder and wasdetermined by subjecting the powder to cut-processing so as to obtain athin film specimen and observation under a transmission electronmicroscope. When the film-like specimen of powder is prepared bycut-processing, the particle size of the powder and the diameter of alow-crystallinity portion in the powder vary. In particular, since thelow-crystallinity portion is not always located in the center of apowder particle, the size of the portion must be observed several timesdepending on the cutting site upon preparation of thin film. Thus, forobservation there were selected particles having a particle size similarto the particle size observed by scanning electron microscopy. Thediameter ratio was determined by observation of 10 or more suchparticles and calculating the average diameter ratio.

[0050] In the “BaTiO₃ powder” column in Table 1, “x”s of the materials(Ba_(1-x)Ca_(x))_(m)TiO₃ powders A to F are 0.00. Thus, powders A to Fcontain no Ca, but powders G to J, in which “x”s are 0.05 or 0.10,contain Ca.

[0051] As additives for the barium titanate powders shown in Table 1,those having the compositions shown in Tables 2 and 3 were provided.Specifically, with respect to Sample Nos. 1 to 10 shown in Table 2, RE(RE represents any one of Gd, Dy, Ho, and Er), Mg and Mn were providedto be added to BaTiO₃ in the form of one of the above-mentioned samplesA to F. A sintering aid containing Si as a primary component was alsoprovided. With respect to Sample Nos. 11 to 19 shown in Table 3, Mg andMn were provided as additives to the (Ba_(1-x)Ca_(x))TiO₃ in the form ofany one of the above-mentioned samples G to J. A sintering aidcontaining (Si, Ti)—Ba as a primary component was also provided. TABLE 2BaTiO₃ + αMg + γMn Si- β γ containing (parts (parts sintering SampleType of α (parts by mole) by by aid (parts No. BaTiO₃ Gd Dy Ho Er mole)mole) by mole) 1 A 0.03 0.030 0.005 3 2 B 0.03 0.010 0.005 5 3 B 0.030.020 0.005 4 4 B 0.03 0.020 0.005 3 5 B 0.03 0.020 0.005 4 6 B 0.030.020 0.005 3 7 C 0.02 0.020 0.020 4 8 D 0.02 0.020 0.005 3 9 E 0.020.020 0.005 3 10 F 0.02 0.020 0.005 3

[0052] TABLE 3 (Si,Ti)-Ba- (Ba_(1−x)Ca_(x))TiO₃ + βMg + γNb containingSample β (parts by γ (parts by sintering aid No. Type of BaTiO₃ mole)mole) (parts by mole) 11 G 0.01 0.005 6 12 G 0.02 0.005 4 13 H 0.020.005 6 14 H 0.02 0.005 4 15 H 0.02 0.005 4 16 H 0.02 0.005 3 17 I 0.020.003 4 18 I 0.02 0.003 4 19 J 0.02 0.005 4

[0053] Respective additives shown in Tables 2 and 3 were transformedinto alkoxide compounds, which are soluble in organic solvent, and thenwere added to the barium titanate powders which had been dispersed in anorganic solvent. Specifically, with respect to Sample Nos. 1 to 10,respective additives were added to the barium titanate powders such that“α,” “β,” “γ,” and “Si-containing sintering aid,” based on parts bymole, in “BaTiO₃+αRE+βMg+γMn” were as shown in Table 2. With respect toSample Nos. 11 to 19, respective additives were added to the bariumtitanate powders such that “β,” “γ,” and “(Si,Ti)αBa-containingsintering aid,” all based on parts by mole, in“(Ba_(1-x)Ca_(x))TiO₃+βMg+γMn” were as shown in Table 3.

[0054] In order to dissolve the above-mentioned additives in an organicsolvent, they may be transformed into alkoxides as described above, ormay be transformed into acetylacetonates or metal soaps.

[0055] The resultant slurries were subjected to evaporation of theorganic solvent to dryness and further heat-treatment, to thereby removethe organic components.

[0056] Subsequently, to each sample of the barium titanate powders towhich respective additives had been added, a polyvinyl butyral binderand organic solvent such as ethanol were added, and the ingredients weresubjected to wet milling so as to prepare a ceramic slurry. Theresultant slurry was molded into a sheet by use of a doctor blade tothereby obtain a rectangular green sheet having a thickness of 1.0 μm.Then, on the resultant ceramic green sheet, a conductive pastecontaining Ni as a primary component was applied by way of printing toform a conductive paste film for forming internal electrodes.

[0057] Subsequently, a plurality of the thus-obtained ceramic greensheets were laminated such that leading ends of the above-mentionedconductive paste films on the sheets were arranged alternately, tothereby obtain a laminate. The resultant laminate was heated at 350° C.in an atmosphere of N₂ so as to burn the binder, and then fired for twohours at the temperature shown in Table 4 in a reducing atmosphere ofH₂—N₂—H₂O gas with a partial pressure of 10⁻⁹ to 10⁻¹² MPa oxygen.

[0058] To the opposite side faces of the fired laminate, a silver pastecontaining B₂O₃—Li₂O—SiO₂—BaO glass frit was applied, followed byburning in a nitrogen atmosphere at 600° C. to obtain externalelectrodes electrically connected with the internal electrodes.

[0059] The outer size of the resultant laminated ceramic capacitor was5.0 mm width, 5.7 mm length and 2.4 mm thickness, and the thickness ofthe dielectric ceramic layer existing between internal electrodes was0.6 μm. The total number of effective dielectric ceramic layers wasfive, and the area of the opposing electrodes per layer was 16.3×10⁻⁶m².

[0060] The electrical properties of the resultant samples were measuredas follows.

[0061] Electrostatic capacity (C) and dielectric loss (tan δ) weremeasured by use of an automatic bridge instrument according to JIS 5102,and dielectric constant (E) was determined by use of the resultantelectrostatic capacity.

[0062] In order to measure insulation resistance (R), an insulationtester was used; by application of 6 V DC for two minutes, insulationresistance (R) at 25° C. was obtained, and resistivity was calculated.

[0063] Regarding the rate of change in electrostatic capacity withrespect to temperature change, the rate of change (ΔC/C₂₀) within arange of −25° C. to +85° C. with reference to the electrostatic capacityat 20° C. and the rate of change (ΔC/C₂₅) within a range of −55° C. to+125° C. with reference to the electrostatic capacity at 25° C. areshown.

[0064] In a high temperature loading test, time-course change ofinsulation resistance upon application of 6 V DC at 150° C. wasmeasured. In this test, the average life of the samples were evaluated,wherein the life of a sample was considered to be equal to time untilbreakdown when the insulation resistance (R) of each sample dropped to10⁵ Ω or less.

[0065] Breakdown voltage was measured by applying DC voltage at avoltage elevation rate of 100 V/sec.

[0066] The average grain size of dielectric ceramic contained in theresultant laminated ceramic capacitor was obtained by chemically etchingpolished cross-sectional surfaces of the laminate and observation of thesurfaces under a scanning microscope. By use of the results and averageparticle sizes of the starting raw materials shown in Table 1, a ratioR, i.e., (average grain size of the dielectric ceramic)/(averageparticle size of the starting raw material) was measured.

[0067] The results are shown in Table 4. TABLE 4 Rate of Rate ofcapacitance capacitance change with respect Dielectric change totemperature Breakdown Firing loss ΔC% change Resistivity voltage AverageSample temperature Size Dielectric tan δ DC3kV/mm ΔC/C₂₀ ΔC/C₂₅ log ρ DClife No. (° C.) ratio R constant (%) (%) (%) (%) (Ω · cm) (kV/mm) (h) 1* 1050 1.55 1260 2.3 −14.6 −10.6 −27.0 13.1 76 0.4  2* 1100 1.76 12803.4 −22.1 −14.6 −30.5 13.1 81 0.5 3 1100 1.04 960 3.6 −3.4 −8.8 −12.413.2 88 65 4 1150 1.08 860 2.8 −3.2 −9.8 −14.7 13.2 94 96 5 1050 1.05910 2.9 −3.3 −9.7 −14.8 13.2 91 88  6* 1050 0.80 630 3.3 −2.4 −12.6−26.7 13.2 88 91 7 1150 1.16 1130 3.6 −4.6 −9.7 −14.8 13.0 91 75 8 11001.10 1040 3.4 −3.8 −8.8 −12.8 13.2 88 63  9* 1150 1.08 1060 3.3 −3.6−9.4 −14.5 13.1 81 4.5 10* 1175 1.04 1670 2.3 −10.9 −9.4 −14.4 13.2 642.1 11* 1100 1.84 1120 3.8 −10.7 2.4 8.7 13.2 68 1.6 12  1100 1.05 7402.6 1.3 −1.4 −4.6 13.2 72 66 13* 1150 1.63 1210 5.7 −11.4 3.4 9.7 13.164 2.5 14  1150 0.97 760 2.3 −2.4 −4.5 −4.5 13.2 96 68 15  1175 1.05 8702.1 −1.5 −4.2 −5.8 13.2 88 64 16* 1150 0.88 230 1.9 2.4 −11.4 −17.9 13.287 96 17* 1250 1.05 1080 4.6 −8.4 −4.2 −8.5 13.2 75 13 18* 1200 0.75 3302.5 1.5 −11.3 −16.4 13.2 83 67 19* 1175 1.04 1030 2.8 −8.6 −2.2 −2.513.2 85 72

[0068] The dielectric ceramic of the present invention is characterizedin that it is obtained by firing barium titanate powder having aperovskite structure in which the c-axis/a-axis ratio in the perovskitestructure is about 1.000 or more and less than about 1.003 and theamount of OH groups in the crystal lattice is about 2.0 wt % or less.Preferably, the barium titanate powder serving as the raw material has amaximum particle size of about 0.3 μm or less, and an average particlesize of about 0.05 to about 0.15 μm. Also, each particle of theabove-described barium titanate powder preferably comprises alow-crystallinity portion and a high-crystallinity portion, the diameterof the low-crystallinity portion being about 0.5 times or more theparticle size of the powder, and a ratio R, i.e., (average grain size ofthe dielectric ceramic)/(average particle size of barium titanatepowder), of about 0.90 to 1.2, so that there is no occurrence ofconsiderable grain growth during sintering ceramic.

[0069] The sample numbers marked with * in Table 4, and the powdersmarked with * in Table 1 fall outside the scope of the present inventionor the above-stated preferable ranges.

[0070] Regarding Sample Nos. 1 to 10 shown in Table 4 obtained throughuse of one of the raw material powders A to F shown in Table 1,transmission electron microscopic analysis of fired ceramic showed thatnear the grain boundary of a ceramic grain, the rare-earth element (RE)such as Gd, Dy, Ho, or Er diffused and formed a shell portion; and, inthe center of the ceramic grain, a core portion was formed; namely, thefired ceramic assumes, within each grain, a core-shell structure inwhich the core and shell have different compositions and crystalsystems.

[0071] As is apparent from the data of Sample No. 1 in Table 4, use of apowder having an amount of OH groups of 2.0 wt % or more, such as thematerial powder A shown in Table 1, is not preferable because thereactivity is too high, making the sintered ceramic grain size large;temperature characteristics of dielectric constant become excessive; andaverage life becomes short.

[0072] In the case of use of material powder such as the material powderE shown in Table 1 having a maximum particle size larger than 0.3 μm, asis apparent from Sample No. 9 in Table 4, the average life of thecapacitor may become short, and, in the case of the present Examplewherein the dielectric ceramic layer has a thickness of 1 μm or less,the reliability may become poor.

[0073] In the case of use of powder material such as the material powderF shown in Table 1 having an average particle size larger than 0.15 μmand a maximum particle size larger than 0.3 μm, as is apparent fromSample No. 10 shown in Table 4, when the dielectric ceramic layer isthin, the reliability may become poor, and electrostatic capacity changeat 3 kV/mm may become excessive.

[0074] As the material powder B shown in Table 1, even in the case inwhich the c-axis/a-axis ratio of the material powder is 1.000 or moreand less than 1.003, the amount of OH groups is 2.0 wt % or less, themaximum particle size is 0.3 μm or less, and the average particle sizeis within a range of 0.05 to 0.15 μm; as is apparent from Sample No. 2shown in Table 4, when ratio R is greater than 1.2, change in thedielectric constant with temperature may become excessive andreliability may become poor.

[0075] As in the case of use of the material powder B shown in Table 1,as is apparent from Sample No. 6 shown in Table 4, when the grain sizeof the sintered body is made small relative to the material particlesize by intensive crushing during ingredient preparation so as toachieve a ratio R of less than 0.90, the dielectric constant may be low,and temperature characteristics of electrostatic capacity may becomepoor.

[0076] In contrast, although in Sample Nos. 3, 4, 5, 7, and 8 shown inTable 4 the thickness of the dielectric ceramic layer is as thin as 0.6μm, the rate of change in electrostatic capacity with temperaturesatisfies the B characteristic specified by JIS specifications withinthe range of −25° C. to +85° C., and satisfies the X7R characteristicspecified by EIA specifications within the range of −55° C. to +125° C.Further, the samples can endure for 60 hours or longer until breakdownoccurs in a high temperature loading test, and they can be fired at1200° C. or lower. The change in electrostatic capacity upon applicationof DC voltage is as small as 5% or less and thus can ensure a highvoltage rating.

[0077] Fired ceramics of Sample Nos. 11 to 19 shown in Table 4 whichwere obtained by use of the material powders G to J shown in Table 1were subjected to transmission electron microscopic analysis. It wasconfirmed that compositional non-uniformity and non-uniform crystalsystem as found for individual ceramic grains in connection with SampleNos. 1 to 17 were not observed.

[0078] The material powders G and H shown in Table 1—in which thec-axis/a-axis ratio of the material powder is 1.000 or more and lessthan 1.003, the amount of OH groups is 2.0 wt % or less, the maximumparticle size is 0.3 μm or less, and the average particle size is withina range of 0.05 to 0.15 μm—shows that the grain size of the ceramicincreases, and in the case in which the particle size ratio R is morethan 1.2, reliability becomes poor and change in electrostatic capacityat 3 kV/mm may becomes large, as is apparent from Sample Nos. 11 to 13in Table 2.

[0079] In the case of use of the material powder H shown in Table 1, asis apparent from Sample No. 16 in Table 4, when the grain size of thesintered body is made small relative to the material particle size byintensive crushing during ingredient preparation so as to achieve aratio R of less than 0.90, the rate of change in electrostatic capacitywith temperature may be large, and relative dielectric constant maybecome poor.

[0080] As the material powder I shown in Table 1, even in the case inwhich average particle size is in excess of 0.15 μm and maximum particlesize is in excess of 0.3 μm, as is apparent from Sample No. 17 in Table4, a ratio R falling within the range of 0.90-1.2 does not necessarilyensure reliability.

[0081] As in the case of use of the material powder I shown in Table 1,as is apparent from Sample No. 18 in Table 4, when the grain size of thesintered body is made small relative to the material particle size byintensive crushing during ingredient preparation so as to achieve aratio R of less than 0.90, the change in electrostatic capacity withtemperature may be large, and relative dielectric constant may becomepoor.

[0082] As is clear from Sample No. 19 in Table 4, when there is used thematerial powder J shown in Table 1 wherein each particle of bariumtitanate powder includes a low-crystallinity portion having a diameter0.5 times or less the particle size of the powder—which indicates that ahigh-crystallinity portion occupies a large area of thepowder—dielectricity might be enhanced and electrostatic capacity at 3kV/mm might increase.

[0083] In contrast, with reference to Sample Nos. 12, 14, and 15 shownin Table 4, although the thickness of the dielectric ceramic layer is asthin as 0.6 μm, the rate of change in electrostatic capacity withtemperature satisfies the B characteristic specified by JISspecifications within the range of −25° C. to +85° C., and satisfies theX7R characteristic specified by EIA specifications within the range of−55° C. to +125° C. Further, the samples can endure for 60 hours orlonger until breakdown occurs in a high temperature loading test, andthe samples can be fired at a temperature of not higher than 1200° C.The electrostatic capacity change upon application of DC voltage is 5%or less and can ensure a high voltage rating.

[0084] As stated above, even if grains of a dielectric have ahomogeneous composition and a uniform crystal system within eachparticle—namely, even if the grains do not have a core-shellstructure—controlling grain growth during sintering can produce adielectric ceramic having excellent temperature-dielectric constantcharacteristic and high reliability, as in the cases of Sample Nos. 12,14, and 15.

[0085] The above-described Example is directed to a laminated ceramicelectronic element in the form of a laminated ceramic capacitor;however, in the case of other laminated ceramic electronic elements suchas a multilayered ceramic substrate which is produced by almost the samemethod, the same results have been confirmed to be obtained.

[0086] As described hereinabove, ferroelectricity of the barium titanatematerial in the dielectric ceramic of the present invention, issuppressed by controlling the c-axis/a-axis ratio in the perovskitestructure and the amount of OH groups in the crystal lattice, preferablyby further controlling the maximum particle size, average particle size,structure of powder particles based on the crystallinity and the graingrowth during firing of barium titanate powder serving as a startingmaterial. Therefore, the dielectric ceramic has an excellenttemperature-dielectric constant characteristic even when the ceramic isused under a strong electric field, and may serve as a dielectricmaterial having small variation in electrostatic capacity and highreliability.

[0087] Accordingly, by use of the dielectric ceramic of the presentinvention, there can be obtained a high-performance laminated ceramicelectronic element having an excellent temperature-dielectric constantcharacteristic, small variation in electrostatic capacity and highreliability. In particular, when the dielectric ceramic is applied to alaminated ceramic electronic element including a laminate in whichdielectric ceramic layers and internal electrodes are superposed one onanother, such as a laminated ceramic capacitor, thetemperature-dielectric constant characteristic can be stabilized even inthe case of a thin ceramic layer having a thickness of 1 μm or less, andthus is advantageous in miniaturizing and thinning the laminated ceramicelectronic element.

[0088] When producing the dielectric ceramic by firing in a reducingatmosphere, the ceramic is not reduced during firing. Therefore, thelaminated ceramic electronic element of the present invention formed byuse of the dielectric ceramic allows use of a base metal such as nickelor a nickel alloy as the internal conductor material, to thereby lowercosts for a laminated ceramic electronic element such as a laminatedceramic capacitor.

[0089] The dielectric ceramic according to the present inventionprovides excellent temperature-dielectric constant characteristics andexcellent reliability, regardless of whether or not it has a core-shellstructure. Therefore, when the dielectric ceramic has no core-shellstructure, the temperature characteristics and reliability are notaffected by the state of dispersion of an additive component, to therebylower the variation in characteristics with firing conditions.

What is claimed is:
 1. A method for producing a dielectric ceramiccomprising a fired barium titanate powder having a c-axis/a-axis ratioperovskite structure of about 1.000 or more and less than about 1.003and an amount of OH groups in the crystal lattice of about 2.0 wt. % orless, which comprises the step of firing a barium titanate powder havinga c-axis/a-axis ratio perovskite structure of about 1.000 or more andless than about 1.003 and an amount of OH groups in the crystal latticeis about 2.0 wt. % or less.
 2. The method for producing a dielectricceramic according to claim 1, wherein the barium titanate powder has amaximum particle size of about 0.3 μm or less and an average particlesize of about 0.05-0.15 μm.
 3. The method for producing a dielectricceramic according to claim 1, wherein individual particles of the bariumtitanate powder comprise a low-crystallinity portion and ahigh-crystallinity portion and the diameter of the low-crystallinityportion is about 0.5 times or more the particle size of the powder. 4.The method for producing a dielectric ceramic according to claim 1,wherein the ratio R which is defined as (average grain size of thedielectric ceramic after firing)/(average particle size of the providedbarium titanate powder) is controlled in the firing step to fall withinthe range of about 0.90-1.2.
 5. The method for producing a dielectricceramic according to claim 1, wherein the powder and firing are suchthat the grains which constitute the dielectric ceramic after firinghave a core-shell structure in which the composition and crystal systemdiffer between the core and the shell.
 6. The method for producing adielectric ceramic according to claim 1, wherein the powder and firingare such that the grains which constitute the dielectric ceramic afterfiring have a homogeneous composition and crystal system.
 7. A methodfor producing a laminated ceramic electronic element comprising aplurality of dielectric ceramic layers comprising a fired dielectricceramic comprising a fired barium titanate powder having a c-axis/a-axisratio perovskite structure of about 1.000 or more and less than about1.003 and an amount of OH groups in the crystal lattice of about 2.0 wt.% or less and an internal conductor on an interface between two adjacentdielectric ceramic layers, which comprises the steps of providing abarium titanate powder having a c-axis/a-axis ratio perovskite structureof about 1.000 or more and less than about 1.003 and an amount of OHgroups in the crystal lattice of about 2.0 wt. % or less; fabricating alaminate in which a plurality of ceramic green sheets comprising thebarium titanate powder and internal electrodes are laminated such thatthe internal electrodes are present at interfaces of the ceramic greensheets; and firing the laminate.
 8. The method for producing a laminatedceramic electronic element according to claim 7, wherein the bariumtitanate powder has a maximum particle size of about 0.3 μm or less andan average particle size of about 0.05-0.15 μm.
 9. The method forproducing a laminated ceramic electronic element according to claim 7,wherein individual particles of the barium titanate powder comprise alow-crystallinity portion and a high-crystallinity portion and thediameter of the low-crystallinity portion is about 0.5 times or more theparticle size of the powder.
 10. The method for producing a laminatedceramic electronic element according to claim 7, wherein the ratio Rwhich is defined as (average grain size of the dielectric ceramic afterfiring)/(average particle size of the provided barium titanate powder)is controlled in the firing step to fall within the range of 0.90-1.2.11. The method for producing a laminated ceramic electronic elementaccording to claim 7, wherein the powder and firing are such that thegrains which constitute the dielectric ceramic after firing have acore-shell structure in which the composition and crystal system differbetween the core and the shell.
 12. The method for producing a laminatedceramic electronic element according to claim 7, wherein the powder andfiring are such that the grains in the dielectric ceramic after firinghave a homogeneous composition and crystal system.
 13. The method forproducing a laminated ceramic electronic element according to claim 7,wherein the internal conductor comprises a base metal.
 14. The methodfor producing a laminated ceramic electronic element according to claim13, wherein the internal conductor comprises nickel or nickel alloy. 15.The method for producing a laminated ceramic electronic elementaccording to claim 7, wherein the step for fabricating a laminatecomprises the sub-steps of providing internal electrodes on interfacesbetween adjacent two of the ceramic green sheets such that one end ofeach of the internal conductors extends to a side face of the laminate;and forming a plurality of external electrodes on the side faces of thelaminate so that the exposed end of each of the internal conductors iselectrically connected to an external electrode.